1. Field of the Invention
This invention is related to a porous body of aluminum or an aluminum alloy (hereinafter referred to simply as an Al material) and a manufacturing method thereof, especially to a porous body of an Al material being excellent in weatherproof property, heatproof property, and mechanical strength and still more being usable as a sound absorbing material which can fully absorb even such a high frequency sound as occurs from a high speed electric car on the Shinkansen as well as various kinds of filters, and a manufacturing method of said porous body.
2. Description of the Prior Art
A porous body made of a sintered metal or alloy of copper powder, iron powder, etc. has been used for a filter. However, it has recently been recognized to be usable as a soundproof material for a high speed railroad.
Namely, it is a well-known fact that recently a high speed electric car of particularly such a railroad as the Shinkansen accompanies not only a requirement for more speed but simultaneously a noise problem from the speed, and to cope with the latter problem some steps are urged to be taken from various aspects. Generally, a so-called sound absorbing material is said to be effective as a counter measure against the noise problem. However, it is required for a sound absorbing material for railroad use to have mechanical strength, heatproof property and weatherproof property, in addition to the sound absorbability. Therefore, no sound absorbing material to meet these demands has appeared yet. Namely, a countermeasure against noise is generally divided into sound interception and sound absorption. The former is to intercept a noise by the so-called intercepting board and the latter is to absorb a noise.
Though many of such sound absorbing materials are mainly made of glass fibers, etc., this kind of sound absorbing material mainly composed of glass fibers is said to have a defect in that it has poor mechanical strength and weatherproof property, and therefore it is especially difficult to install on a vehicle which is to run or vibrate, and difficult to position to directly absorb the noise from the source of sound itself, because the sound absorbing material is easy to break by an outer power such as an impact.
Under these circumstances, a porous alloy sintered body especially composed of a copper alloy has been given attention recently as a sound absorbing material because it is excellent in mechanical strength, weatherproof property and sound absorbability. In particular such a sound absorbing material has high mechanical strength and zigzag connecting pores therein. Thus, noises are considered to be absorbed because wave motion energy of the noises is changed into heat energy while they pass through said connecting pores.
However, a sound absorbing material of such construction is considerably restricted in its actual use because it is very expensive and heavy since said material is usually composed of a copper alloy group. This is said to be a defect of this sound absorbing material.